using Newtonsoft.Json.Linq;
using System.Collections.Generic;
using System.Net.Http;

namespace app1
{
    class Program
    {
        static void Main(string[] args)
        {
            var url = "https://api.dominio.com.br/login";
            var userName = "fulano@gmail.com";
            var password = "12345678";

            var token = GetToken(url, userName, password);
        }

        static string GetToken(string url, string userName, string password)
        {
            var pairs = new List<KeyValuePair<string, string>>
                    {
                        new KeyValuePair<string, string>( "username", userName ),
                        new KeyValuePair<string, string> ( "password", password )
                    };
            var content = new FormUrlEncodedContent(pairs);

            using (var client = new HttpClient())
            {
                var response = client.PostAsync(url, content).Result;
                JObject o = JObject.Parse(response.Content.ReadAsStringAsync().Result);
                return (string)o["token"];
            }
        }
    }
}